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Mass of a person

  1. Aug 30, 2009 #1

    Can someone tell me how to find out the mass of a person. If someone weighs 150 lbs (on Earth) what is their mass? Any chance someone can point me to a web page that will convert weight to mass for me.

  2. jcsd
  3. Aug 30, 2009 #2
    Weight is a force. f = ma. a on earth is g = -9.8m/s^2. m = F/g.
  4. Aug 31, 2009 #3


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    In the usual units, the mass of something on earth has the same numerical value as its weight on earth. Anything which weighs 150 lbs. on earth will have a mass of 150 lbs. If you ever take a physics course they will try to set up separate units for mass and weight, but it can be confusing.
  5. Aug 31, 2009 #4


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    It's a little simple in metric. In imperial units pounds are used for force and mass.

    A 150lb (mass) person is about 68kg (mass)
    On earth they have a weight (force) of f = mg = 68 * 9.8 = 666N
  6. Aug 31, 2009 #5
    Can I use lbs for F or do I have to use kgs?

  7. Aug 31, 2009 #6
    But your weight will change if you're on the moon, mars, etc... Your mass is always the same regardles of where you are.
  8. Aug 31, 2009 #7

    (force) = (mass) (accelleration due to gravity)

    Like mathman said a pound is actually also unit a for mass but the SI unit which is usually used is kg.

    If you want to use pounds for force you're actually using lbf (pound-force) units.

    In physics courses they typically use Newtons (= kg x m/s^2) for force.

    btw Technically "weight" is the magnitude of the force vector, which changes depending on the local gravity, but people sometimes use the word interchangeably with mass since with the lbf units they are the same on Earth.
  9. Sep 1, 2009 #8


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    You can't use kgs, that measures mass not force. If you are working in "mks" (the "meters, kilograms, seconds" version of the metric system), force is measured in "Newtons" as mgb_phys said. In the English system "pounds" is a force, in the laboratory, the "slug" (the mass that a force of 1 pound would accelerate at 1 foot per second squared) is used for mass.
  10. Sep 1, 2009 #9
    OK, I think there is some confusion in what I am asking. I step on a scale in my bathroom and it says I weigh 175 lbs or using the metric system 79.5 kg. If I'm on mars, venus, pluto, etc... my weight will be different. But my mass, which I believe is measured in KG is always the same no matter where I weigh myself. What I want to know is the formula for finding out my mass given my weight on Earth. Now most formulas use the metric system so I'm assuming I can't input my weight in pounds, I have to use KG also.

    So what is the forumal for getting my mass given my weight on Earth and what unit of measure do I have to use for weight?

  11. Sep 2, 2009 #10
    Those scales factor in the 9.81 m/s of gravity, they are measuring weight but giving a mass output.

    If the scales say 79.5 kg, then you technically 'weigh' 779 Newtons.

    Basically earth scales are onlyvalid on earth, as they mesure weight then divide by 9.81 to give the reasong in SI units.

    However you can convert the units to imperial or english if you want, the relationship holds.

    F = mg thats all it is.

    Pounds are (stupidly) used for both mass and force, they are NOT the same value however.

    1 lbf (force) = 1lb (mass) * 32.17405ft/s^s (gravity)

    so 1lbf = 32.17 lb.ft/s^2.

    What your scales weigh if the give a reading of 175 lbs is actually 5629 lbf(force) and then divide by the value of gravity to give your mass of 175 lb(mass).

    So this is the point, you dont 'weigh' 79 kg or 175 lb. That is your mass. The scales have done the calculation for you.
  12. Sep 2, 2009 #11
    So the Earth's gravity varies at different locations and if I record my mass using the the scales at these locations.
    Won't the scales give a different value for my mass depending on location.
    If the scales had done the calculation the value for the mass would not vary.
  13. Sep 2, 2009 #12


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    Yes for accuracy better than 1% you need to recalibrate the scales depending on your location.
    From the equator to the poles your weight varies by about 0.5% and local geology also has a slightly smaller effect.
  14. Sep 2, 2009 #13
    The scales will give a different value for mass becuase they are calibrated to a specific gravity. In localised regions of higher/lower gravity the reading will be off because your weight will change but the scales dont 'know' the gravity has changed.

    The weight to mass calcualtion is just a constant multiplier that either has to be programmed in on a digital scale or the wheel has to be calibrated on an analogue scale.
  15. Sep 5, 2009 #14
    So where do you decide is the correct location to give the accurate reading for both mass and weight.
    In other words where is the place it is all nailed down.
    Who decided and why.
  16. Sep 5, 2009 #15


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    Weight and mass aren't really defined like that.
    Mass is fixed an independant of position, so if you measure the weight (ie force) at a particular location - that gives you the local value of 'g' = the acceleration due to gravity.
  17. Sep 6, 2009 #16
    Mass never changes.

    The value of 'g' used to calculate weight is taken from an average of different 'g' values from different points.

    This is the reason we assign a standard value for gravity.

    Its also the same reason we assign a standard value for atmospheric pressure, that vaires but for the sake of ease of calulation a value is chosen that minimises error at all locations.
  18. Sep 6, 2009 #17
    So if mass never changes how is it fixed independently of position.
    Presumably it has to be calculated at some place or every place using fixed instruments which have been calibrated with different 'g' values.
  19. Sep 6, 2009 #18
    Mass is independent of g, it is an inherent property of the object in question.

    There are numerous ways to determine mass without having to adjust calculations for local values of g. Here are a few:
    • Apply a known force and measure the resulting acceleration
    • Attach the object to a spring and measure its frequency of vibration
    • Attach the object to a rotor (or string) and measure the amount of centripetal force required to swing it in a circle at a given velocity
  20. Sep 6, 2009 #19


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    The mass is fixed it's a block of platinium in Paris that is (roughly) the same mass as 1000cc of water (the original definition of the Kg)
    This block would weigh different amounts in different places - but if you compare it's weight with another identical block (with just a simple see-saw balance) that would also be the same anywhere and wouldn't depend on the weight (or the details of the balance)

    If you used one of these blocks to calibrate a weighing scale you would have to do that where you are using the scale - by putting a 1kg block on the scale and marking where it registered 1kg (or 981N). In practice for a high accuracy digital lab scale you might just calibrate it as the factory (in known g) and supply a table of adjusted g for each city.
  21. Sep 7, 2009 #20


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    I was overseas recently, and noticed on a packet of sugar, it said Net Weight 5g. I commented to the person with me that this was technically wrong. It should say net mass. But I kept seeing this convention over and over again, using kg and g as units of weight. This was true in different English-speaking countries outside the US.

    So I looked it up on the internet. Wikipedia has an article that says that kg and g not only represent kilograms and grams, but they also represents kilogram-force and gram-force, the force a mass of 1 kg or 1 g exerts under an acceleration of Earth's surface gravity.

    Is anyone familiar with this? It makes sense, since people all over the world speak of their weight, not mass, and most of the English-speaking world outside the US and UK don't use pounds for anything.
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